Robotic devices and systems are finding increased usage in a variety of applications. They extend into the realms of medicine (e.g. prosthetic devices), military applications, industrial robots (for assembly), hazardous industrial environments and so on.
For a robotic device to operate intelligently within a given but flexible and changing environment, it must be able to accurately determine, or sense, what its surroundings are. Advanced sensory capabilities will characterize the next generation of robots, and among these sensory functions is tactile sensing, the ability to determine physical features through touch mechanisms. Although the goal can be stated quite simply, the technological implementation presents quite another challenge. Furthermore, the tactile sensing capability is a broad spectrum: at one end of the spectrum is the ability to merely detect the presence of an object, and at the other end is the ability to determine the surface texture of an object. Rounding out the spectrum is the ability to determine an object's size and shape and whether or not it has moved on the sensor's surface.
An example of a piezoelectric device is U.S. Pat. No. 4,328,441 (Kroeger, et al) and its international counterpart W0 No. 81/02223. These reveal a layered structure having piezoelectric polymer films on opposite sides of an insulating layer for the purpose of providing a keyboard. This does not avoid the phantom point problem.
IBM Technical Bulletin, Vol. 20, No. 1, J. P. Dahl, June 1977, reveals a scanned piezoelectric keyboard switch where each key is chosen to have a unique inherent resonant frequency while the switches are wired in parallel. Contact dampens the frequency and the impedance of the undampened crystal changes greatly.
IBM Technical Bulletin, Vol. 20, No. 7, J. Fajans, December 1977, discloses an acoustical touch panel in which acoustic plane wave impulses are generated at fixed times in orthogonal directions by two long piezoelectric crystals mounted on adjacent sides of a lower plate. Local acoustical coupling is said to result in a spherical wave originating from the point of contact in an upper plate when an impulse is present in the lower plate.
P. Dario et al, "Touch Sensitive Polymer Skin Uses Piezoelectric Properties to Recognize Orientation of Objects", an article in Sensor Review p. 194-198, Oct. 1982, use a single layer polyvinylidene fluoride PVF.sub.2 sensor with 256 sensing areas (16.times.16 array) to recognize object orientation. One lead pin is required for each sensing area.
A bilaminate PVF.sub.2 sensor is proposed in "Piezo-Pyroelectric Polymers Skin-Like Tactile Sensors for Robots and Prostheses", 13th Symposium 7 Conference and Exposition on Industrial Robots and Robots, Chicago, R. Bardelli et al, April 1983, where the outer layer senses temperature and the inner senses mechanical forces. The article teaches against row by column reading involving multiplexing. A lead for each sensing area is advocated.
In "Piezoelectric Polymers: New Sensor Materials for Robotic Applications", 13th Symposium on Industrial Robots and Robots 7 Conference and Exposition Chicago, P. Dario et al April 1983, various PVF.sub.2 contact sensors and touch sensors are described. The touch sensor using 256 sensor regions has at least 256 leads. A tactile sensor using a PVF.sub.2 emitter and receiver uses the time of flight of ultrasonic waves through a compliant material to measure pressure on the sensor.
In the prior art there are several major disadvantages that are overcome by the present invention. First the location, shape, and pressure of an object can be actively sensed. Secondly, switching noise problems are overcome by multiplexing the energizing signal rather than the sensing signal. Third, the invention avoids the "phantom point" problem of a crossed array. Fourth, great sensitivity and high resolution are possible. Finally, the complexity of the lead array can be greatly reduced by allowing the use of N+M leads to address N.times.M active areas.